Leucine Metabolism of Streptococcus Lactis VAR. Maltigenes. II. Transaminase and Decarboxylase Activity of Acetone Powders

The cause of the malt-like flavor and aroma in milk cultures is 3-methyl-butanaI, which originates from leucine.The authors have studied the physiological characteristics of the reaction involved in establishing the metabolic pathway by which leucine is degraded to 3-methylbutanal by cultures of Streptococcus lactis var.maltigenes.Editor.The precursor of 3-methylbutanal, the compound responsible for the malt-like aroma and flavor of milk cultures of Streptococcus lactis var.maltigenes, has been shown to be leucine (4).Resting cells of the organism are able to convert a-ketoisocaproic acid to both 3-methylbutanal and leucine (6).The present work was undertaken to demonstrate the presence of active lcucine-glutamic transaminase in acetone powders of S. lactis and its malty variant, and to elucidate the a-ketoisocaproic decarboxylative mechanism of S. lactis var.maltigenes thereby establishing a metabolic pathway by which leucine is degraded to 3-methylbutanal. EXPERIMENTAL PROCEDUREPreparation of acetone powders.Packed washed cells of each organism were collected as described previously (6) and suspended in 15-25 ml. of cold (4 ° C.) distilled water, and 150-200 ml. of cold (-10 ° C.) acetone was added (1).The suspensions were shaken vigorously and quickly 25-30 times and then centrifuged at -2 ° C. at 2,200 r.p.m, for approximately 45 minutes.The packed cells were transferred quickly to chilled weighed petri dishes, then placed in a chilled desiccator, and dried under vacuum over HfSQ, and finally over P:O~ at 4 ° C.After drying (24-48 hours), the residues were ground in a chilled mortar, weighed, and stored in air-tight containers at -10 ° C. The yield from 6-10 ml. of packed cells was 1.0-1.5 g. of dried powder. Dialysis procedure.For the manometric studies dialyzed powders were employed.The powders were taken up in 15 ml. of cold 0.033 M phosphate buffer at pH 4.6; the suspensions were sealed in cellophane tubing and suspended in